U.S. patent number 7,167,101 [Application Number 11/353,400] was granted by the patent office on 2007-01-23 for method and apparatus for telemetry.
This patent grant is currently assigned to Halliburton Energy Services, Inc.. Invention is credited to Paul D. Beene, Christopher A. Golla, Laban M. Marsh, Bipin K. Pillai, Paul F. Rodney, Cili Sun.
United States Patent |
7,167,101 |
Golla , et al. |
January 23, 2007 |
Method and apparatus for telemetry
Abstract
A method and related apparatus for telemetry between downhole
devices and surface devices. In particular, the methods and related
apparatus may send a first datum of a first parameter in an
uncompressed form, and send a second datum of the first parameter
in compressed form.
Inventors: |
Golla; Christopher A.
(Kingwood, TX), Marsh; Laban M. (Houston, TX), Rodney;
Paul F. (Spring, TX), Sun; Cili (Sugar Land, TX),
Pillai; Bipin K. (Tomball, TX), Beene; Paul D.
(Kingwood, TX) |
Assignee: |
Halliburton Energy Services,
Inc. (Houston, TX)
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Family
ID: |
38372171 |
Appl.
No.: |
11/353,400 |
Filed: |
February 14, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060232438 A1 |
Oct 19, 2006 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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11105754 |
Apr 14, 2005 |
7106210 |
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Current U.S.
Class: |
340/853.1;
166/250.01; 340/853.2; 367/83; 375/222; 455/72; 702/6 |
Current CPC
Class: |
E21B
47/18 (20130101); G01V 11/002 (20130101) |
Current International
Class: |
G01V
3/00 (20060101) |
Field of
Search: |
;340/853.1,853.2
;375/222 ;367/83 ;702/6 ;166/250.01 ;455/72 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wong; Albert K.
Attorney, Agent or Firm: Scott; Mark E. Conley Rose,
P.C.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Continuation-In-Part Application of, and
claims priority to, "Method and Apparatus for Mud Pulse Telemetry,"
application Ser. No. 11/105,754, now U.S. Pat. No. 7,106,210 filed
Apr. 14, 2005, incorporated herein by reference herein as if
reproduced in full below.
Claims
What is claimed is:
1. A method comprising: sending a first datum of a first parameter
in uncompressed form from a surface unit to a downhole unit; and
sending a second datum of the first parameter in compressed form
from the surface unit to the downhole unit within the mud column by
sending a first delta value being a difference between the first
and second datum.
2. The method as defined in claim 1 further comprising: receiving
the uncompressed first datum and the compressed second datum by a
downhole device; and reconstructing the second datum from the first
datum and the first delta value.
3. The method as defined in claim 2 further comprising: sending a
second delta value, being a difference between the second datum and
a third datum of the first parameter; and reconstructing the third
datum from the first datum, the first delta value and the second
delta value.
4. The method as defined in claim 2 further comprising: sending a
second delta value, being a difference between the first datum and
a third datum of the first parameter; and reconstructing the third
datum from the first datum and the second delta value.
5. The method as defined in claim 2, wherein sending the first
delta value further comprises encoding a most likely value of the
first delta value as a zero.
6. The method as defined in claim 5 further comprising encoding a
second most likely first delta value as one of a value of one and a
value of two.
7. The method as defined in claim 2 further comprising selecting a
number of bits to use to encode the first delta value based on the
size of the first delta value.
8. The method as defined in claim 1, wherein sending the second
datum further comprises sending a plurality of compressed data of
the first parameter, each of the plurality of compressed data
related to the uncompressed datum.
9. The method as defined in claim 8, wherein the number of data
points of the plurality of compressed data are selected, at least
in part, on a bit error rate of communications from the surface
unit.
10. The method as defined in claim 1, wherein sending the first
datum occurs prior to sending any other datum in the first
parameter.
11. The method as defined in claim 1, wherein sending the first
datum further comprises encoding the first datum based on a most
likely value of the first datum.
12. A method of transferring data values in a telemetry system,
comprising: sending a predetermined value to a downhole device;
sending a plurality of compressed values to the downhole device,
wherein each compressed value is the difference between an
uncompressed value and the predetermined value; and calculating the
uncompressed values of the compressed values using the
predetermined value.
13. The method as defined in claim 12, wherein sending the
predetermined value comprises sending the predetermined value prior
to sending the plurality of compressed values.
14. The method as defined in claim 13, wherein at least one of
sending the predetermined value or sending the plurality of
compressed values further comprises introducing a series of
pressure pulses into a fluid being pumped without interrupting the
pumping.
15. A system, comprising: a surface unit; and a downhole assembly
communicatively coupled to the surface unit; wherein the surface
unit transmits a group of compressed datums to the downhole
assembly, wherein at least one of the compressed datums is a
difference between a predetermined value and an uncompressed datum
corresponding to said at least one of the compressed datums;
wherein the downhole assembly determines uncompressed values of the
compressed datums using the predetermined value.
16. The system of claim 15, wherein the surface unit transmits the
predetermined value to the downhole assembly prior to transmission
of the group of compressed datums.
17. The system of claim 16, wherein the surface unit transmits the
predetermined value as an uncompressed value to the downhole
assembly prior to transmission of the group of compressed
datums.
18. The system of claim 15, wherein the predetermined value is
stored on at least one of the surface unit or the downhole assembly
prior to transmission of the group of compressed datums.
19. The system of claim 15, wherein the surface unit transmits a
group of compressed datums to the downhole assembly using a series
of pressure pulses in a fluid.
Description
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
Embodiments of the invention are directed to telemetry in drilling
operations. More particularly, embodiments of the invention are
directed to data compression techniques for telemetry in drilling
operations.
2. Background of the Invention
In measuring-while-drilling (MWD) and logging-while-drilling (LWD)
operations, information regarding the borehole and surrounding
formation are gathered during the drilling process. Information
gathered may not be needed at the surface immediately, but that
information may be required before the tool returns to the surface.
For information such as this, U.S. Pat. No. 5,774,420 may describe
a system whereby stored data (also known as historical data) may be
sent from downhole devices to the surface at the request of the
surface equipment. Retrieval of the historical information may take
place during times when drilling is temporarily paused, such as
when the borehole is being conditioned (e.g. by the continuous flow
of drilling fluid), or when the tool becomes stuck in the borehole.
Transmission of historical information from downhole to the surface
may take several hours using known techniques.
Other information gathered downhole may be needed at the surface as
soon as the information is acquired. A limiting factor in sending
data from downhole devices to the surface (or for that matter from
the surface to downhole devices) is the speed at which the
information may be transmitted within the mud column. Where the
acquisition rate by the downhole device is greater than the
transmission rate, some of the information gathered downhole may
not be sent to the surface. In cases such as this, it may be that
only every other or every third reading of the "real time"
parameter may be sent to the surface.
Thus, what is needed in the art is a mechanism to speed the
effective transmission rate of information in a mud pulse telemetry
system.
BRIEF DESCRIPTION OF THE DRAWINGS
For a detailed description of the preferred embodiments of the
invention, reference will now be made to the accompanying drawings
in which:
FIG. 1 shows a drilling system in accordance with embodiments of
the invention;
FIG. 2 shows a graph of ideal pressure pulses in drilling
fluid;
FIG. 3 shows a more realistic graph of pressure pulses in drilling
fluid in accordance with embodiments of the invention; and
FIG. 4 shows a graph of average bits per second versus data bits in
a list with no compression, and with 1:1 compression.
NOTATION AND NOMENCLATURE
Certain terms are used throughout the following description and
claims to refer to particular system components. This document does
not intend to distinguish between components that differ in name
but not function.
In the following discussion and in the claims, the terms
"including" and "comprising" are used in an open-ended fashion, and
thus should be interpreted to mean "including, but not limited to .
. . ". Also, the term "couple" or "couples" is intended to mean
either an indirect or direct connection. Thus, if a first device
couples to a second device, that connection may be through a direct
connection, or through an indirect connection via other devices and
connections. Further, in some embodiments, the term "uncompressed"
may be used in reference to a datum or other value that is
entropy-encoded.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The various embodiments of the present invention were developed in
the context of hydrocarbon drilling operations sending information
from downhole devices to the surface through mud pulse telemetry
techniques. Because of the developmental context, this
specification explains the concepts in terms of data transmission
from downhole devices to the surface; however, this patent should
not be construed as limited only to the precise developmental
context, as the systems and methods may be useful in other
applications, and using other telemetry methods.
FIG. 1 shows an embodiment of a drilling system having a drill
string 10 disposed within a borehole 12. The drill string 10 has at
its lower end a bottomhole assembly 14 which may comprise a drill
bit 16, downhole measuring and/or logging devices 18, and a
transmitter or pulser in a mud pulse communication system 20. The
downhole sensors 18 may comprise any now existing or
after-developed logging-while-drilling (LWD) or
measuring-while-drilling (MWD) devices or tools. The bottomhole
assembly 14 may also comprise systems to facilitate deviated
drilling such as a mud motor with bent housing, rotary steerable
systems, and the like. Moreover, the lower end of the drill string
10 may also comprise drill collars (not specifically shown) to
assist in maintaining the weight on the bit 16. Drill string 10 is
preferably fluidly coupled to the mud pump 22 through a swivel 24.
The swivel 24 allows the drilling fluid to be pumped into the drill
string, even when the drill string is rotating as part of the
drilling process. After passing through bit 16, or possibly
bypassing bit 16 through pulser 20, the drilling fluid returns to
the surface through the annulus 26. In alternative embodiments, the
bottomhole assembly 14 may mechanically and fluidly couple to the
surface by way of coiled tubing; however, the methods of
compressing information for transmission described in this patent
may remain unchanged.
Embodiments of the invention may transmit data gathered by downhole
tools to the surface by inducing pressure pulses into the drilling
fluid--mud pulse telemetry. In particular, the drill string 10 may
comprise mud pulse communication system 20 that couples within the
drill string, and also couples to the measuring and/or logging
devices 18. The mud pulse communication system may thus gather data
from the devices 18, and transmit the data to the surface by
creating mud pulses in the drilling fluid within the drill string.
Mud pulse telemetry may also be used to transmit a variety of data
from the surface to downhole devices. For example, mud pulse
telemetry may be used to transmit instructions from the surface
used to guide the drill bit 16 while drilling. Mud pulse telemetry
also may be used to transmit signals from the surface to downhole
devices used to acknowledge the receipt of drilling data
transferred from the devices 18 to the surface.
FIG. 2 shows an exemplary graph of drilling fluid pressure as a
function of time, which may be measured by the signal processor 28
coupled to the pressure sensing device 30 (FIG. 1) for
communications from downhole devices to the surface, or which may
be measured by the communication system 20 for communications from
surface devices to downhole devices. The exemplary graph of FIG. 2
represents an ideal situation where ideal square wave pulses are
generated, and are detected as ideal square waves. In actual
systems, this may not be the case. However, FIG. 2 may help
identify terminology related to the various embodiments. In
particular, FIG. 2 illustrates that a "list" may comprise a
plurality of "intervals," e.g. list 32 comprising three intervals
I.sub.1, I.sub.2 and I.sub.3. An interval may be the time duration
between the leading (or alternatively trailing) edges of
pulses.
FIG. 3 shows a more realistic graph of pressure pulses, as may be
detected by pressure sensor 30 and signal processor 28, or
communication system 20. Rather than being the ideal square wave
pulses as depicted in FIG. 2, these pulses may be dampened, may
have their frequency components dispersed, and the like. FIG. 3 may
also help exemplify several parameters of a pulse position
modulation system. Interval I.sub.1 is shown to have a particular
time length or duration. The duration of the interval I.sub.1 is
preferably longer than a maximum interval length of the remaining
intervals in each list so that the start of the new list may be
identified. In alternative embodiments, a long interval may reside
at the end of the list. For each remaining interval, such as
I.sub.2 and I.sub.3 (whether data encoded is a list identification
number, actual data gathered by downhole sensors 18, or
instructions from the surface signal processor 28), there is a
minimum time (MIN-TIME) for the interval. An interval having a
length substantially equal to the MIN-TIME encodes a data value of
zero. FIG. 3 exemplifies, in the second interval, two pulses having
a MIN-TIME duration and that may represent a data value zero. The
MIN-TIME may range from between approximately 0.3 seconds and 2.0
seconds for most drilling systems, with a MIN-TIME of 0.6 seconds
preferred. The MIN-TIME duration may need to be greater than
approximately three times a pulse duration ("D" of FIG. 2), where
the pulse duration is the time duration of a pulse event. A pulse
event may be either a positive pulse or a negative pulse, for
example created by transmitter 20.
FIG. 3 also exemplifies that the interval duration need not
necessarily be precise to represent a value. Instead, the
embodiments of the invention may utilize a window in which a pulse
of an interval may fall, yet still represent the same value. For
the second interval of FIG. 3, the second pulse 36 may fall within
the BIT-WIDTH window. So long as a pulse falls within its BIT-WIDTH
window, the data value encoded may still be the same. In the
particular example of pulse 36, the interval may represent a data
value of zero. The BIT-WIDTH window, however, is applicable to each
received pulse in the pulse train. For example, the pulse 38 drawn
in dashed lines falls within the next BIT-WIDTH window, and
therefore the time duration between pulse 35 and pulse 38 may
represent a data value of one. Likewise, the pulse 40 falls within
the third BIT-WIDTH window, and therefore the time duration between
pulse 35 and pulse 40 may represent a data value of two. In more
general terms, the value encoded in the pulse position modulation
system may be decoded using substantially the following equation:
DATA=(INTERVAL-MIN-TIME)/BIT-WIDTH (1) Wherein DATA is the decoded
value, INTERVAL is the measured time of the interval, and MIN-TIME
and BIT-WIDTH are as described above. Given existing technology,
BIT-WIDTH values may range from approximately 0.03 seconds to 0.12
seconds; however, a BIT-WIDTH value of 0.04 seconds is preferred.
For a particular number of bits encoded within each interval, there
is a maximum time (MAX-TIME) length or duration. For example, if a
particular interval encodes a four-bit number (which could
therefore range in value from zero to fifteen), the four-bit number
at its maximum value forces an interval duration equal to its
MAX-TIME. Co-pending application Ser. No. 10/305,529 titled "Data
Recovery for Pulse Telemetry Using Pulse Position Modulation," and
co-pending application Ser. No. 10/306,487 titled "Structure and
Method for Pulse Telemetry," both of which are incorporated by
reference herein as if reproduced in full below, describe methods
and systems for mud pulse telemetry, including error detection and
correction, that may be utilized in various embodiments of the
invention.
Embodiments of the invention group intervals into lists. For
example, list 32 and list 34 in FIG. 2 each comprise three
intervals. For communication from downhole devices to the surface,
each list may comprise values of detected downhole parameters such
as, without limitation, uncompressed electromagnetic wave
resistivity (an eight-bit value encoded in two intervals), an
uncompressed gamma ray reading (an eight-bit value encoded in two
intervals), and an uncompressed density value (a twelve bit value
encoded in three intervals). For communications from the surface to
downhole devices, each list may comprise values such as directional
data. Multiple lists may be created. The following table
exemplifies the components of a group of intervals forming an
uncompressed list in accordance with embodiments of the
invention.
TABLE-US-00001 TABLE 1 Bit Number Interval 7 6 5 4 3 2 1 0 1 PAD2
PAD1 PAD0 P4 P3 P2 P1 P0 2 0 0 0 0 ID3 ID2 ID1 ID0 3 0 0 0 0 A7 A5
A3 A1 4 0 0 0 0 A6 A4 A2 A0 5 0 0 0 0 B7 B5 B3 B1 6 0 0 0 0 B6 B4
B2 B0 7 0 0 0 0 C3 C2 C1 C0 8 0 0 0 0 C7 C6 C5 C4 9 0 0 0 0 C11 C10
C9 C8
In Table 1 (PAD 2 . . . PAD 0) are pad bits in the long interval
that may be selectively set to ensure the long interval is longer
than MAX-TIME of the remaining intervals, and thus identifies the
start of a new list, (P4 . . . P0) are parity bits calculated using
the encoded data contained in the list, (ID3 . . . ID0) are
identification bits which identify the list, and therefore the data
values in the list, (A7 . . . A0) are bits of an exemplary eight
bit uncompressed downhole parameter, (B7 . . . B0) are bits of an
exemplary eight bit uncompressed downhole parameter, and (C11 . . .
C0) are the bits of an exemplary twelve bit uncompressed downhole
parameter. Table 1 exemplifies that in the preferred embodiments,
except for the initial interval, the intervals in a list have
encoded therein a number of bits that is less than the number of
parity bits, and may be the same for each interval. The number of
bits in each data interval may be selected to increase efficiency
of the transmission time given a particular BIT-WIDTH and MN-TIME.
For most applications, identification and data intervals using four
bit encoding are preferred. Table 1 shows only the transfer of
three pieces of uncompressed data (two eight bit parameters and a
twelve bit parameter); however, any number of related or unrelated
parameters may be transferred within any one list.
Because of the speed at which downhole devices traverse the
formations in MWD and LWD systems, formation and/or borehole
parameter values may not rapidly change between readings taken by
downhole devices. Based on this fact, and possibly in order to
increase an effective data transmission rate in a mud pulse
telemetry system, various embodiments of the invention may utilize
a data compression method when transmitting the data uphole. As
described further below, the data compression method may also be
used when transmitting data from the surface to downhole devices.
By compressing the data prior to its transmission, it may be
possible to reduce the overall number of bits of information which
need to be sent uphole or downhole relative to the same amount of
uncompressed data, thus increasing effective data rate.
While there may be many possible data compression methods that may
be utilized, the preferred embodiments use a Delta value
compression system on data. Consider for purposes of explanation,
and with reference to Table 1 above, three exemplary types of
telemetry data A, B and C. As illustrated in Table 1, data type A
may be an eight-bit parameter, data type B may likewise be an
eight-bit parameter, and data type C may be a twelve-bit parameter.
In the related art, each of these parameters A, B and C may be
transmitted to the surface in full, uncompressed format, regardless
of the amount of change (if any) in value between the previous
transmission and the current transmission. The various embodiments
of the present invention, however, on at least some occasions
encode a compressed version of each of the data types for
transmission. For example, if parameter A has experienced no change
in value from the value that was previously transmitted to the
surface, then in the preferred embodiments only a data value of
zero may be sent (rather than encoding again the entire eight bit
value). Likewise, if the parameter A experiences only a small
change in value from the value previously sent, a number
representing the change in value may be transmitted to the surface.
This change in value, or Delta value, may require fewer bits;
therefore, the overall number of bits to transfer the information
is reduced, increasing the effective data throughput. An example
using real numbers may be helpful in understanding the Delta value
concept.
Consider for purposes of explanation only, a downhole tool having
an eight bit parameter with the following sequence of data to be
transmitted to the surface: 110, 112, 115, 111 and 107. In one
embodiment, the first datum or value transmitted may be in its
uncompressed, eight bit format. For some number of intervals
thereafter, only the changes in value from the uncompressed datum
may be sent. In this example, the values transmitted may be: 110,
+2, +5, +1, and -3. In embodiments of the invention, the compressed
values may be related to the immediately prior value, whether
compressed or uncompressed. Thus, in these embodiments, the
transmitted values for the number sequence above may be: 110, +2,
+3, 4, and 4.
In more mathematical terms, Delta values may relate back to the
previous uncompressed value according to the following equation:
.DELTA.A[n]=A[n]-A[m] (1) where A is the downhole parameter of
interest, .DELTA.A is the change in value of parameter A, n is the
index to the current datum, and m is the index to the last
uncompressed datum transmitted. Likewise with respect to the
embodiments where Delta values relate to the immediately previously
sent value, the Delta values may relate to each other according to
the following equation: .DELTA.A[n]=A[n]-A[n-1] (2)
Selecting one of the compression methods of equations (1) or (2)
above may be based on the bit error rate of the particular system.
A bit error rate may be a relationship between a number of bits
transmitted to the surface, and a number of bits correctly received
and decoded by surface equipment. In mud pulse telemetry systems
where the bit error rate is relatively low (a system experiencing
low corruption of data in the transmission process) for example,
having Delta values relate back to the immediately previous value
(equation (2)) may be utilized. The Delta modulation of equation
(2) may be used with low telemetry bit error rates because a bit
error that corrupts a set of data (a bit error that is not
correctable) may cause all values thereafter to not be usable. By
contrast, the Delta modulation method that relates the Delta value
back to the last uncompressed value (equation (1)) may be more
desirable in situations where bit error rates are high. In this
system, loss of any particular Delta value does not affect the
calculation of actual values based on subsequently transmitted
Delta values.
The number of bits used to encode Delta values may be based on the
relative size of the Delta values as well as the number of bits
encoded in each interval. In at least some of embodiments of the
invention, the compressed values transmitted to the surface (or
from the surface to downhole devices) may be encoded using a number
of bits related to the number of bits in the intervals in the list.
As exemplified in Table 1, each of the intervals after the long
interval may encode four bit values. With the preferred short or
data interval width of four bits, the Delta value for an eight-bit
value may be encoded within a single interval, comprising four
bits. Likewise, the Delta value for a twelve bit parameter may be
encoded in either four bits (one interval), or eight bits (two
intervals).
Using exemplary parameters A, B and C from Table 1 above, the Delta
value companion list to the list of Table 1 may read as
follows:
TABLE-US-00002 TABLE 2 Bit Number Interval 7 6 5 4 3 2 1 0 1 PAD2
PAD1 PAD0 P4 P3 P2 P1 P0 2 0 0 0 0 ID'3 ID'2 ID'1 ID'0 3 0 0 0 0
.DELTA.A3 .DELTA.A2 .DELTA.A1 .DELTA.A0 4 0 0 0 0 .DELTA.B3
.DELTA.B2 .DELTA.B1 .DELTA.B0 5 0 0 0 0 .DELTA.C3 .DELTA.C2
.DELTA.C1 .DELTA.C0 6 0 0 0 0 .DELTA.C7 .DELTA.C6 .DELTA.C5
.DELTA.C4
Where ID' may identify the companion list to an uncompressed list.
Thus, rather than encoding the uncompressed values of each of the
parameters A, B and C as exemplified in Table 1, Table 2 shows that
the overall list may comprise Delta values for each of the
parameters A, B and C. With Delta values encoded as four-bit
numbers for each of the parameters, the list may be shortened from
nine total intervals (Table 1) to only six intervals. A receiving
device (e.g., surface computer, such as signal processor 28 of FIG.
1 or a downhole device such as communication system 20), may
calculate actual values of the exemplary three parameters by the
decoding the information using one of either the previous
uncompressed list or the previous compressed list, depending upon
the compression method.
At least some of the parameters sent are in a compressed,
preferably Delta modulated, format. One possible encoding mechanism
is to directly encode the Delta values within the interval. For
example, if the Delta value is +1, and the interval width is four
bits, it would be possible to encode a binary [0001] to indicate
the +1 Delta value. Likewise, if the Delta value is +2, one
possible implementation would be to encode the value [0010] in the
interval. As for negative values, for example -2, the leading bit
in the interval could be set to indicate a negative value, such
that -2 may be encoded as [1010], or alternatively a 1's-compliment
may be used and therefore encoding of value [1101]. While each of
these encoding methods, as well as others, may be operational, the
preferred embodiments utilize an encoding method for the Delta
values that may, on average, shorten the compressed data interval
length, and therefore further decrease transmission time.
If the Delta values for a particular transmitted value are tracked
on a statistical basis, a probability of any particular Delta value
occurring may take a normal distribution centered at zero. In other
words, the most likely Delta value may be zero. The next most
likely Delta values may be small positive and negative values near
zero, for example, +1 and -1, and the like. A Delta value of zero
may be encoded within an interval as a zero value, thus the
interval will have only a MIN-TIME duration. With regard to the
remaining possible Delta values, the preferred embodiments may
utilize a method called "entropy encoding." In entropy encoding,
the most likely or most probable values to send (Delta values or
otherwise), and regardless of their actual value, are assigned
smaller binary values, and therefore the shorter transmission times
in a pulse position modulation system. Table 3 below shows an
exemplary assignment of integer Delta values and their
corresponding bit patterns within each interval.
TABLE-US-00003 TABLE 3 .DELTA. Value Encoded Value 0 0000 +1 0001
-1 0010 +2 0011 -2 0100 +3 0101 -3 0110 +4 0111 -4 1000 +5 1001 -5
1010 +6 1011 -6 1100 +7 1101 -7 1110 +8 1111
As shown in illustrative Table 3, the most probable Delta value may
have an encoded value of zero. The second most likely Delta value
may have encoded values of binary [0001] (for +1) and binary [0010]
(for -1) respectively--values having only one and two bit widths
respectively longer pulse time than the MIN-TIME. Although Table 2
shows integer Delta values, this need not necessarily be the case.
For example, a bulk density reading may span 1.2 to 3.2 grams per
cubic centimeter in normal logging operations, and because of the
resolution of the downhole device, the Delta values may be 0.0,
+0.02, -0.02, +0.04, -0.04, and so on. Using the entropy encoding
techniques, the +0.02 Delta value may be assigned an encoded value
of binary [0001]. Likewise, the Delta value of -0.02 may be
assigned an encoded value of binary [0010], and the like.
The Delta compression technique as embodied in equations (1) and
(2) above comprises determining Delta values for at least part of a
list of data values based on the last uncompressed datum
transmitted in the list. However, the scope of disclosure is not
limited to determining Delta values as such. Instead, in some
embodiments, these Delta values may be determined based on a value
other than the last uncompressed datum transmitted. By determining
the Delta values based on a value other than the last uncompressed
datum transmitted, an entire list, including the first value in the
list, may be compressed. For example, in some embodiments, the
Delta values may be determined based on a value in a
previously-transmitted list. In other embodiments, a predetermined
value may be programmed into the mud pulse communication system 20,
and Delta values for at least part of a list of data values may be
determined based on this predetermined value. Alternatively, the
predetermined value may be transmitted to the mud pulse
communication system 20 from the signal processor 28 before or
during drilling.
For communications from downhole devices to the surface, the
predetermined value may be selected based upon the expected range
of numerical values that may be measured downhole. For
communications from the surface to downhole devices, the
predetermined value may be selected based upon the expected range
of numerical values that may be transmitted. For instance, a
predetermined value of 1.0 may be selected when it is expected that
downhole measurements shall produce numerical values ranging from
+0.5 to +1.5. The predetermined value may likewise be selected
based upon the resolution of the downhole device. The scope of
disclosure is not limited to selecting the predetermined value in
any particular manner. Instead, the predetermined value may be
selected using a variety of suitable techniques.
Embodiments of the invention may use many compression ratios
depending on the bit error rate of the system: 1:1 compression (one
compressed list for each uncompressed list), a 1:2 compression (two
compressed lists for each uncompressed list), and so on. In mud
pulse telemetry systems having high bit error rates, where many
intervals have errors that are uncorrectable, 1:1 compression may
be the most advantageous. In yet other systems where the bit error
rate is relatively low, higher compression rates 1:M (where M is
the number of compressed lists for each uncompressed list) may be
used. For example, in communications from the downhole devices to
the surface, a downhole device may send an uncompressed list of
parameters, and thereafter send a series of compressed lists up to
the predetermined M. After M compressed lists have been sent, the
downhole system may again send an uncompressed list. The sending
device need not, however, stringently follow the desired
compression rate.
The various embodiments of the invention may also have the
capability to refrain from sending a compressed list when any one
of the Delta values exceeds a number that may be encoded in the
number of bits in a compressed interval. In this circumstance, the
sending device may send an uncompressed version of the parameters,
and then attempt in the next interval to send compressed values.
Thus, if 1:3 compression is being utilized in an exemplary system,
and a Delta value for one of the parameters in what should be the
second compressed list exceeds that which may be encoded in a
compressed interval, the sending device reverts to sending an
uncompressed list, and resets a counter so that the subsequent
three intervals may be sent in compressed format (Delta values
allowing). Even if only a 1:1 compression ratio is used, however,
the effective transmission rate may still increase.
In the non-limiting case of an uncompressed list comprising two
eight bit parameters and one twelve bit parameter, a total of forty
bits of information (including pad bits, parity bits and list
identification bits) may be sent. If those same three parameters
have their Delta values sent rather than their uncompressed values,
and each Delta value for the eight-bit parameters may span only
four bits and each Delta value for the twelve bit parameter may
span only eight bits (as exemplified in Table 2), it is possible
that only twenty-eight total bits may be needed to transmit the
Delta values to the surface. FIG. 4 shows the average number of
bits per second transmitted in the system as a function of the
total number of data bits in each list. The first series 42 shows
the average number of bits per second with no compression (each
list sent in uncompressed format). The second series 44 exemplifies
the effective number of bits per second that may be seen in the
system utilizing a 1:1 compression. As is exemplified in FIG. 4,
even a 1:1 compression may result in statistically significant
increases in the effective bits per second transmitted.
As described in Table 1 above, each list may have a list
identification number comprising, in at least some embodiments,
four bits. Because of this number of bits, the list identification
number may thus take on sixteen possible states. In order to
identify uncompressed lists and their companion compressed lists,
embodiments of the invention determine, possibly prior to
deployment, the list identification numbers of the uncompressed
lists, as well as their companion compressed lists. Using Tables
(1) and (2) as an example, Table (1) may be an uncompressed list
having a list identification ID. Table (2) may be a companion
(compressed) list having list identification ID'. For example, and
without limitation, a first uncompressed list may be assigned a
list identification number of zero, and its companion compressed
list may be assigned binary [1111].
The various embodiments described to this point have assumed
multiple parameters contained in each list, and that each parameter
may likewise have a corresponding compressed version that may be
sent in a compressed list. Given the speed at which information may
be transmitted in a mud column, it may be possible that multiple
downhole parameters may be sampled or determined in the amount of
time that it takes one set of information to be transmitted to the
surface. In other words, downhole tools may calculate borehole and
formation parameters faster than a list may be telemetered to the
surface in uncompressed form. Although surface equipment may be
receiving "real time" data, the surface may only be receiving every
other or every third datum. In alternative embodiments, it is not
necessary that each list contain different parameters, and instead
each list may contain multiple readings of the same parameter. The
compression technology discussed in this specification may,
therefore, be used to increase the volume of data for intervals
comprising data for a single parameter sent uphole or downhole. For
example, a list comprising nine intervals may be modified such that
it contains one uncompressed value, and then a plurality of
compressed or Delta values based, either directly or indirectly, on
the uncompressed value. A plurality of subsequent lists may contain
only compressed values, for example. The number of subsequent lists
containing compressed values is related to the particular
compression ratio used for the system. In this way, surface
equipment may be able to receive all the data generated downhole
for particular parameters.
Relatedly, in some embodiments, the data compression may allow
interleaving such that if any one list is corrupted and
uncorrectable, the surface system may still have data spanning that
period of time. More particularly, a first list may send values of
parameter A of A[N], .DELTA.A[N+2], .DELTA.A[N+4] and the like. A
subsequent list may thus carry datums of the A parameter of A[N+1],
.DELTA.A[N+3], .DELTA.A[N+5] and the like. If either the first list
or the second list has an uncorrectable bit error, the receiving
system still has valid data from that period of time. It is noted
that in this example each list contained an uncompressed datum and
a plurality of compressed datums; however, a subsequent list need
not have the uncompressed values as discussed above. As an
alternative to this interleaving, subsequent lists may overlap data
so that should any one list experience an uncorrectable bit error,
the data spanning the time period may be reconstructed from the
immediately prior and subsequent lists. For example, consider four
lists having the following data: List 1--A[N], A[N+l], A[N+2],
A[N+3]; List 2--A[N+1], A[N+2], A[N+3], A[N+4]; List 3--A[N+3],
A[N+4], A[N+5], A[N+6]; List 4--A[N+4], A[N+5], A[N+6], A[N+7].
Thus, should either of lists 2 or 3 have uncorrectable bit errors,
no data will be lost.
Other methods may be used to reduce data loss given uncorrectable
bit errors in transmission. Consider a series of three lists: a
first list having an uncompressed value (and possibly compressed
values); a second list having compressed values relating back to
the uncompressed value in the first list; and a third list having
an uncompressed value. If there is no correlation between the
second and third list, an uncorrectable bit error in the first list
renders the first and second list unusable. However, in at least
some embodiments, one of the compressed values of the second list
may correlate to the uncompressed value in the third list. For
example, the last compressed value may be the same value as will be
sent as the uncompressed in the third list. In this way, should the
first list be lost to uncorrectable bit errors, the second list may
still be used by back-calculating the values using the uncompressed
value from the third list.
In embodiments of the invention where surface equipment receives
real-time data of a plurality of different parameters in each list,
time tagging of data, possibly for correlating the data to depth,
may take place at the surface. That is, surface equipment, such as
a processor, may note the time the data was received, then
back-calculate when the downhole samples were taken by accounting
for travel time of the pulses within the mud column and signal
processing latencies in the downhole equipment. In embodiments of
the invention where each list contains a plurality of values of the
same downhole parameter, the sample time calculated at the surface
may not be applicable to each value in the list, as these values
may not have been simultaneously determined. In cases such as this,
at least some embodiments of the invention order the data in the
lists such that the last datum corresponds to the last sample
taken. The time calculated by surface equipment, again possibly
taking into account travel time of the mud pulses in the mud column
and down hole processing latencies, may thus be associated with the
last datum, and time tags for remaining values in the list may be
calculated by knowing the periodicity at which samples of the
parameter of interest are taken down hole.
In alternative embodiments of the invention, downhole samples may
have been taken many minutes or hours from when they are
transmitted to the surface, and thus may be referred to as
"historical data." Time tagging data values of the same parameter
in a list in these embodiments may involve sending a list
containing a start time or time tag for a first datum. The list
containing start time may be sent a plurality of times to ensure
that the surface equipment receives the information. Thereafter, a
plurality of lists may be sent to the surface, each list comprising
data of the parameter. Each list may additionally comprise a
counter value that identifies each of the samples in the list in
relation to the first datum (possibly in a previous list). Surface
equipment, knowing the start time of the data, the periodicity of
the samples, and a sample number for each datum, may thus calculate
a time tag for each datum. While sending the start time or time tag
for the first datum prior to sending the remaining is preferred,
the list containing the time tag may be sent before, during or
after the bulk of the data. Further, while sending the lists with
data in sample order may be preferred, the lists may be sent in any
order given that the counter value may identify a sample number of
each datum in the list without reference to counter values from
other lists.
Although not necessarily required, the preferred embodiments of the
present invention implement a smoothing function on the downhole
data prior to its transmission to the surface. Such smoothing may
also be used in downhole communications. The inventors of the
present specification have found that smoothing does not unduly
affect the accuracy of the downhole parameters, and further the
smoothing aids in removing noise from the downhole parameters that
may cause an unnecessarily large number of, or unnecessarily large,
Delta values for any particular parameter. Although many smoothing
functions may be utilized, e.g. averaging over a time window,
averaging over N number of points, in the preferred embodiments,
"exponential smoothing" is utilized using substantially the
following equation.
.alpha..alpha. ##EQU00001## where y is the smoothed datum of a
particular index i, x is the raw datum of a particular index and
.alpha. is a smoothing coefficient that varies with the resolution
of the tool and the rate of penetration. Any value above zero may
be used, with .alpha. of 0.5 being preferred.
Many of the various embodiments described to this point have
assumed downhole-to-surface communications. However, the Delta
compression and entropy encoding techniques described above may
also be used to facilitate communications from the surface to
downhole devices. Such communications may comprise, for example,
instructions sent from a surface computer (e.g., the signal
processor 28) to the bottomhole assembly 14 that direct the
drilling direction of the drill bit 16. Communications from the
surface to downhole devices may also comprise signals that
acknowledge receipt of drilling data transmitted from downhole
devices to the surface. The scope of disclosure is not limited to
transmitting any particular type of data uphole or downhole, and
the Delta compression and entropy encoding techniques may be used
on any data suitable for compression and/or encoding. Delta
compression and entropy encoding techniques of surface-to-downhole
transmissions may be performed using a variety of hardware, such as
hardware described in "Downlink Telemetry System," U.S. Pat. No.
6,920,085, which is incorporated herein by reference.
The above discussion is meant to be illustrative of the principles
and various embodiments of the present invention. Numerous
variations and modifications will become apparent to those skilled
in the art once the above disclosure is fully appreciated. For
example, it is possible that compressed data and unrelated
uncompressed data may be contained within the same list. A primary
list may have uncompressed values, and a companion list may have
compressed values for some of the parameters, but also contain one
or more uncompressed values. Further, the specification has
discussed that compressed values should be encoded using four bits;
however, any number of bits may be used for the Delta values
without departing from the scope and spirit of the invention.
Moreover, it may be possible that an interval of a list may contain
multiple compressed values, for example, two, two-bit Delta values
may be encoded together in a four-bit interval. Further still, it
is contemplated that downhole system or systems may be capable of
switching between Delta values having varying resolutions. Thus, in
the case of Delta values for a single parameter contained within an
interval, the downhole system may use Delta values having two bits
when the size of the Delta values so allows, and the downhole
device may switch to Delta values encoded using four bits if the
Delta values so require. The resolution of use may be identified by
the companion list ID number. Though the specification has
described the compression in the context of mud pulses, the
compression techniques described may find application in any form
of MWD and LWD communications, such as electromagnetic and
acoustic. Furthermore, combinations of technologies may be used,
e.g. mud pulse and electromagnetic could be used at the same time.
The data compression could be used across all channels, or merely
subsets of the channels. The communication systems described are
equally applicable to communication from surface devices to
downhole devices. Finally, entropy encoding need not necessarily be
tied to Delta values, and instead could apply to any value,
including a value of first interval of a list. In these
embodiments, the most likely values of the particular parameter
could be worked out in advance, enabling entropy encoding on
substantially any value. It is intended that the following claims
be interpreted to embrace all such variations and
modifications.
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